Compound X is a weak acid that has two ionizable protons. The pKa values for the

Question

Compound X is a weak acid that has two ionizable protons. The pKa values for the

protons are 2.5 and 7.2. The molecule starts out with a net charge of +2 at pH 2 and below and

the charge is due to the protonation state of the ionizable protons. What is the likely net charge

at pH 6? pH 10? You can use approximations for your answer. Uncharged molecules are more

diffusive in biological systems through lipid bilayer membranes. Assuming that blood is buffered

at pH 7.4 will this molecule be more or less diffusive across biological membranes in higher

animals? No need for a concrete number here

Explanation / Answer

The compound X has two ionizable protons.

And it is said to have a charge of +2 for the pH 2 which is below the first pKa value of the acid.

Thus, at pH = 6;

pH>pKa1

thus one proton is unprotonated above pKa1 = 2.5

Thus, net charge at pH = 6 is +1.

At pH = 10, pH> pKa2 and thus the other remaining proton is also unprotonated and hence the net charge will be 0.

In blood which is buffered at a pH of 7.4, the pH > pKa2 (7.2) and hence the protons are not protonated and the net charge on the molecule is 0. Thus, the molecule is uncharged and will be more diffusive across biological membranes.

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